For the purpose of providing added value to lenses of this type, different designs have been developed over the years. These designs seek to correct aberrations perceived by the wearer due, among others, to placing the lens in a specific position and inclination with respect to the eye. To that end monofocal designs which try to correct those possible aberrations introduce variations into the geometry of the sphero-toric surface for the purpose of reducing said aberrations of the lens or equivalently increasing wearer comfort. These modifications can be, for example, introducing aspherical, aspherotoric or atoric surfaces to decrease oblique astigmatism; or the inclusion of a small addition in the lens to improve comfort in near vision.
The literature shows various possibilities for designing the new surfaces, the most common being those generated by conic curves:c(y2+(1+Q)x2)−2x=0where c is the curvature of the surface at the source and Q is the asphericity. Depending on the value of Q it is possible to generate hyperbolic surfaces (Q<−1), parabolic surfaces (Q=−1), spherical surfaces (Q=0) and surfaces in the form of ellipsoids (−1<Q<0 or Q>0). With said conic curves it is possible to reduce different types of aberrations (oblique astigmatism, curvature of field, distortion, magnification, etc.) which are generated due to the position of the lens with respect to the eye.
Prior studies have established an angle α of 30° as a limit for the line of sight displacement with respect to the neutral position (corresponding to 0°), after which the wearer accompanies with head movements for the rotation to be made. Therefore, conic curves are generally used to minimize the weight of the aberrations taking into account a cone of vision forming the gaze direction of the wearer at 30°, because it is also known that if a 30° angle is corrected, all angles less than that have a considerably low aberration level. Even still, these optimizations are normally limited to certain prescription or base curve ranges or to cutting one or both surfaces of the lens.
Documents U.S. Pat. Nos. 3,960,442, 5,083,859, 5,825,454, US 2006/0132708 and ES 2,337,970 describe various examples.